Laser devices have been heretofore suggested and/or utilized, and such lasers have included gas lasers (see, for example, U.S. Pat. No. 3,428,914) and multi-mode lasers (see, for example, U.S. Pat. No. 4,089,964). In addition, open cavity gas lasers have heretofore been suggested and/or utilized, particularly in the field of particle size measurements (see, for example, U.S. Pat. No. 3,406,289 wherein a device is described for sizing particles using extinction of laser energy). Refinements in extinction particle size measurement utilizing open cavity laser devices were also described by R. G. Knollenberg and B. Schuster in "Detection and Sizing of Small Particles in Open Cavity Gas Laser", Applied Optics, Volume 11, Number 7, November, 1972, pages 1515-1520.
Submicron particle sizing devices utilizing light scattering in an open cavity laser device was first described by R. G. Knollenberg in "An Active Scattering Aerosol Spectrometer", Atmospheric Technology, Number 2, June, 1973, pages 80-81. Refinements have been described by R. G. Knollenberg in "Active Scattering Aerosol Spectrometry", National Bureau of Standards Special Publication 412, issued October 1974, pages 57-64; by R. G. Knollenberg and R. E. Luehr in "Open Cavity Laser `Active` Scattering Particle Spectrometry from 0.05 to 5 Microns", Fine Particles, Aerosol, Generation Measurement, Sampling and Analysis, Editor Benjamin Y. H. Liu, Academic Press, May, 1975, pages 669-696; by R. G. Knollenberg in "Three New Instruments for Cloud Physics Measurements: The 2-D Spectrometer, the Forward Scattering Spectrometer Probe, and the Active Scattering Aerosol Spectrometer", American Meteorological Society, International Conference on Cloud Physics, July, 1976, pages 554-561; and by R. G. Knollenberg in "The Use of Low Power Lasers in Particle Size Spectrometry", Proceedings of the Society of Photo-Optical Instrumentation Engineers: Practical Applications of Low Power Lasers, Volume 92, Aug. 1976, pages 137-152.
While open cavity gas lasers typically have high Q values (the ratio of the resonant energy density inside a laser cavity to all possible losses is referred to as the "Q" of the cavity), such lasers have presented difficulties in utilization, and this has led to attempts to develop external cavities fed by standard lasers in an attempt to overcome these difficulties. The application of external cavities fed by standard lasers has been heretofore limited, however, due to the low Q values attained, which low Q values have been largely due to the lack of exact phase locking and competition with the cavity having gain.
Attempts have heretofore also been made to frequency stabilize the active cavity of a laser device, and such attempts have included position adjustment of a frequency defining member of the cavity. Such devices are shown, for example, in U.S. Pat. Nos. 3,534,289, 3,718,868 and 3,899,748.